The present invention relates to a method of measuring the concentration of toner contained in a two-component developer applicable to a developing device of an electrophotographic electrostatic or image forming apparatus, and an apparatus therefor.
To develop a latent image electrostatically formed on an image carrier, use is made of either a one-component developer, i.e., toner or a two-component developer which is a mixture of toner and carrier. Regarding the two-component developer, toner and carrier are each charged to a particular polarity by being agitated, i.e., by friction. The charged toner develops a latent image, or electrostatic charge image, having been charged to the opposite polarity to the toner. In this connection, in reversal development, the toner deposits on exposed portions. There are available a magnet brush method using iron powder as carrier, a cascade method using beads as carrier, a fur brush method and so forth, which are selectively used in matching relation to the kinds of toner and carrier.
The management of the toner concentration of the developer is vital in insuring high quality and stable images. The toner concentration of the developer is measured by, for example, a magnetic bridge type sensor which is responsive to the magnetic permeability of the developer. A toner concentration is estimated on the basis of a relation between a toner concentration and a sensor output and then fed back. This kind of scheme is disclosed in, for example, Japanese Patent Laid-Open Publication No. 4-24651. However, when the fluidity of the developer stored in a developing device decreases, the developer cannot be agitated stably. As a result, the sensor fails to operate stably and, in the worst case, outputs a signal not representing the actual toner concentration.